The influence of Ce doping of titania on the

The influence of Ce doping of titania on the photodegradation of phenol

Marcela V. Martin, Paula I. Villabrille & Janina A. Rosso

Environmental Science and Pollution Research ISSN 0944-1344 Environ Sci Pollut Res DOI 10.1007/s11356-015-4667-4

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Author's personal copy Environ Sci Pollut Res DOI 10.1007/s11356-015-4667-4

RESEARCH ARTICLE

The influence of Ce doping of titania on the photodegradation of phenol Marcela V. Martin 1 & Paula I. Villabrille 2 & Janina A. Rosso 1

Received: 29 December 2014 / Accepted: 6 May 2015 # Springer-Verlag Berlin Heidelberg 2015

Abstract Pure and cerium-doped [0.05, 0.1, 0.3, 0.5, and 1.0 Ce nominal atomic % (at.%)] TiO2 was synthesized by the sol–gel method. The obtained catalysts were characterized by X-ray diffraction (XRD), UV–visible diffused reflectance spectroscopy (DRS), Raman, and BET surface area measurement. The photocatalytic activity of synthesized samples for the oxidative degradation of phenol in aqueous suspension was investigated. The content of Ce in the catalysts increases both the transition temperature for anatase to rutile phase transformation and the specific surface area, and decreases the crystallite size of anatase phase, the crystallinity, and the band gap energy value. The material with higher efficiency corresponds to 0.1 Ce nominal at.%. Under irradiation with 350 nm lamps, the degradation of phenol could be described as an exponential trend, with an apparent rate constant of (9.1 ± 0.6) 10−3 s−1 (r2 =0.98). Hydroquinone was identified as the main intermediate.

Responsible editor: Philippe Garrigues * Janina A. Rosso [email protected] Marcela V. Martin [email protected] Paula I. Villabrille [email protected] 1

Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA) CONICET, Universidad Nacional de La Plata (UNLP), La Plata 1900, Argentina

2

Centro de Investigación y Desarrollo en Ciencias Aplicadas (CINDECA) CONICET, Universidad Nacional de La Plata(UNLP), La Plata 1900, Argentina

Keywords Photocatalysis . Doped titanium dioxide . Cerium . Phenol

Introduction Phenolic compounds constitute an important family of wastewater pollutants produced by chemical, petrochemical, foodprocessing, or biotechnological industries (Patel et al. 2014). The current levels of pollutant removal from water, with conventional water treatment technologies, are often not fully satisfactory when wastewater streams contain significant amounts of hardly biodegradable compounds such as phenolic pollutants (Adán et al. 2011). In recent years, the application of heterogeneous photocatalytic water purification processes has gained wide attention due to its effectiveness in degrading and mineralizing the recalcitrant organic compounds as well as the possibility of utilizing the solar UV and visible-light spectrum. By far, TiO2 has played a much larger role in this scenario compared to other semiconductor photocatalysts due to its low cost, effectiveness, inert nature, and photostability. The photocatalytic mechanism is initiated by the absorption of the photon hν with energy equal to or greater than the band gap of TiO2 (~3.1 eV for the anatase phase) producing an electron-hole pair on the surface of TiO2 particle. An electron is promoted to the conduction band (CB), while a positive hole is formed in the valence band (VB). Excited state electrons and holes can recombine and dissipate the input energy as heat, get trapped in metastable surface states, or react with electron donors and electron acceptors adsorbed on the semiconductor surface or within the surrounding electrical double layer of the charged particles. After the reaction with water, these holes can produce hydroxyl radicals with high redox oxidizing potential. Depending upon the exact conditions, the holes, OH, O2 −,

Author's personal copy Environ Sci Pollut Res

H2O2, and O2 itself can play important roles in the photocatalytic reaction mechanism (Grabowska et al. 2012). A substantial amount of research has focused on the enhancement of TiO2 photocatalysis by modification with metal, nonmetal, and ion doping (Pelaez et al. 2012; Grabowska et al. 2012; Daghrir et al. 2013). The primary purposes of doping TiO2 are to retard the fast charge recombination and enable visible light absorption by creating defect states in the band gap. Generally speaking, the photocatalytic activities of TiO2 depend on various parameters in a complex way. Not only the intrinsic properties of TiO2 (e.g., crystallinity, surface area, particle size, defect sites, surface charge) but also the substrate–surface interactions that should depend on the kind of substrate play significant roles in deciding the overall photocatalysis (Park et al. 2013). Doping with lanthanides is a promising choice since lanthanide ions are known for their ability to form complexes with various Lewis bases (e.g., alcohols, aldehydes, amines, etc.) through the interaction of these functional groups with the f orbitals of lanthanides (Ranjit et al. 2001). Moreover, rare earth oxides are found to have polymorphs, strong adsorption selectivity, good thermal stability, etc., due to their f electron and multi-electron configuration (Xu et al. 2006). Particularly, the utilization of Ce-doped TiO2 was reported for the degradation of some substrates: 2mercaptobenzothiazole (Li et al. 2005), rhodamine B (Xiao et al. 2006), formaldehyde (Xu et al. 2006), 4chlorophenol (Silva et al. 2009), methylene blue (Aman et al. 2012), and bisphenol A (Chang and Liu 2014). The efficiencies of these treatments were diverse, as expected, because the process is substrate-dependent. Several reviews focus on comparing the efficiency of different treatments for the degradation of phenol as a Bmodel^ contaminant (Esplugas et al. 2002; Liotta et al. 2009; Grabowska et al. 2012). However, no reports were found about the influence of cerium doping of TiO 2 on the photodegradation of phenol. We present here a comparative study on the photocatalytic efficiency of undoped and Ce-doped TiO2 (0.05–1.0 Ce nominal at.%) prepared by the sol–gel method. The synthesized samples were characterized by X-ray diffraction, Raman scattering, UV-visible diffuse reflectance spectra, and N 2 physisorption. The photocatalytic activity of the materials for the oxidative degradation of phenol in aqueous suspension, under different irradiation sources, was investigated.

(Anedra), nitric acid (Anedra), titanium dioxide (Degussa P25), and phenol (Aldrich). The metal-ion precursor used in the preparation was cerium(III) nitrate hexahydrate (Ce(NO3)3.6H2O, Aldrich). Preparation of photocatalysts The photocatalysts were prepared by standard sol-gel methods. TiO2 sols were obtained by dropwise addition of an ethanolic TTIP solution, where 5 mL of TTIP was mixed with 50 mL of anhydrous ethanol, to 50 mL of distilled water adjusted to pH 1 with nitric acid under vigorous stirring at 25 °C. After continuously stirring for 24 h, the resulting suspension was evaporated and dried at 50 °C overnight. The obtained crystals were ground to powder and calcined at 400 °C (6 °C min−1) or 600 °C (6 °C min−1) for 1 h, under air. Ce-doped TiO2 samples were prepared according to the above procedure in the presence of Ce(NO3)3 6H2O to give a doping level from 0.05 to 1.0 nominal at.%. The appropriate amount of metal-ion precursor was added to distilled water before the hydrolysis of TTIP, and the subsequent procedures were the same as described above. The undoped sample is white, while the doped samples exhibited different tones of yellow according to their Ce content. The prepared materials calcined at 600 °C will be referred to hereafter as undoped and 0.05 Ce, 0.1 Ce, 0.3 Ce, 0.5 Ce, and 1.0 Ce (referring to Ce nominal at.%). Similarly, the catalysts calcined at 400 °C will be referred to as undoped-400 and 0.1 Ce-400. Characterization of the photocatalysts Crystal structure patterns of the Ce-doped TiO2 powder samples were examined by X-ray diffraction (XRD) using a Philips diffractometer (PW-1390) with Cu Kα radiation. The Raman scattering measurements of TiO2 samples were performed in the backscattering geometry at room temperature in air using a Jobin-Yvon T64000 triple spectrometer, equipped with a confocal microscope and a nitrogen-cooled chargecoupled device detector. The spectra were excited by a 514.5 nm line of argon laser with an output power of less than 5 mW to avoid local heating due to laser irradiation. BrunauerEmmett-Teller (BET) surface area measurements were carried out using N2 as the adsorptive gas (Micromeritics ASAP 2020). UV–Vis diffuse reflectance spectra (DRS) were obtained on a UV–Vis Perkin-Elmer Lambda 35 spectrophotometer. Photocatalytic decomposition of pollutant

Experimental section Chemicals The following specific reagents were used in this study: titanium tetraisopropoxide (TTIP, Aldrich), absolute ethanol

The photocatalytic oxidation of phenol in catalyst suspensions was carried out in a glass reactor at 25 °C, in air and with continuous stirring to ensure that the material was in suspension. For each run, the reaction mixture was prepared by ultrasonically dispersing 100 mg catalyst, 5 μg phenol


Results and discussion Catalyst characterization Figure 2 shows XRD patterns of undoped and modified TiO2 photocatalysts. It was found that all samples calcined at 400 °C were of anatase phase. Compared to highly crystalline anatase TiO2, (103) and (112) crystal faces did not grow very well, which commonly occurred in TiO2 calcined below 500 °C (Sun et al. 2013). An increase in calcination temperature to 600 °C resulted in a significant improvement in the crystallinity of the material. For undoped titania, a complete A–R (anatase to rutile) transformation was observed. The presence of both phases, anatase (A) and rutile (R), in doped material with a low nominal amount of Ce (0.05 Ce, 0.1 Ce, and 0.3 Ce) was detected, while for 0.5 Ce and 1.0 Ce, only the A phase was observed. This result indicates that the presence of Ce inhibited the phase transformation at 600 °C. The chemical bonds of Ti– O–Ce three elements around the anatase crystallites could easily occur during the thermal treatment process, which possibly inhibited the formation and growth of the crystal nucleus of rutile (Liqiang et al. 2004).

Intensity / u.a.

Light sources 575 nm white 350 nm

glass absorption region

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Fig. 1 Emission spectra of light sources and reactor glass absorption region

Moreover, as the content of Ce increased in the doped samples, the peaks broadened, and their intensity slightly weakened indicating weaker crystallinity. Lanthanide ions are known to be less reactive than titania precursor species, which also slows down the condensation and crystallization processes of the titania matrix (Aman et al. 2012). No diffraction peaks that could be attributed to the dopant ion were observed, not even for the highest Ce content. Our result suggests that the doping levels or the thermal treatment employed does not induce the formation of discrete impurity phases of cerium oxide, in accordance with a previous report (Aman et al. 2012). Furthermore, the ionic radius of Ce3+ is 0.097, which is much larger than that ofTi4+ (0.061 nm) but smaller than that of oxygen (0.14 nm). Hence, it is difficult for lanthanide ions to enter the TiO2 lattice to replace Ti4+ ions (Nguyen-Phan et al. 2009). It is conceivable that metal impurities, which were formed during synthesis, were nanoscopic or possibly well dispersed on the surface (Choi et al. 2010).

A R

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Intensity / a.u.

(Aldrich), and 100 mL ultrapure water (Milli-Q: resistivity >18 MΩ cm and 400 nm increases with the Ce content (see Fig. 5), the irradiation with specific sources of light did not yield a better efficiency of photocatalytic degradation. This observation is in line with the behavior described by other authors (Li et al. 2005; Choi et al. 2010). However, due to the relevance of visible light in the solar spectrum, the degradation of 25 % of the initial concentration of phenol by 0.1 Ce is a promising result. For the three lamps used, the addition of a low amount of cerium to the catalysts (0.05 %) increases the percentage of degradation of phenol compared to undoped material. However, when the 0.1 Ce at.% (0.6 % w/w) is exceeded, the efficiency stays the same or decreases. This decrease in the photocatalytic activity of Ce-doped catalysts with the increment of Ce concentration above certain values was already

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undoped TiO2 300

350

400

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 / nm Fig. 5 UV–visible diffuse reflectance spectra (DRS) for the synthesized catalysts (calcined at 600 °C)

observed and was attributed to the presence of amorphous phase, promoting electron-hole recombination (Silva et al. 2009). We found that 0.05 Ce, 0.1 Ce, and 0.3 Ce with both phases (A and R) were better materials for the degradation of phenol. A similar observation, namely that the mixtures of both phases exhibit higher photoactivity as well as effective degradation of phenol in comparison with pure A or R catalysts, was reported for pure TiO2 (Zhang et al. 2000). Moreover, it was reported (Choi et al. 2010) that metal-doped TiO2 with a relatively high fraction of rutile (around 28 %) showed significantly enhanced photocatalytic activity (evaluated for methylene blue degradation, iodide oxidation, and phenol degradation under visible-light irradiation at λ >400 nm). Recently, Wang et al. (2014) reported that an increase in the crystallite size from 6.6 to 26.6 nm led to a significant improvement in the photoreactivity of pristine anatase nanoparticles (for the degradation of phenol), while this reactivity was

10

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4 20

Concentration /  M

Increment of % at. Ce

independent of the crystallinity. Interestingly, our best performing catalysts (0.05 Ce, 0.1 Ce and 0.3 Ce) have A crystallite sizes (see Table 1) similar to the better value reported there (Wang et al. 2014). Therefore, the relation of A/R phases seems to be as important as the crystallite size in the photoreactivity of TiO2 catalysts. A closer study was performed with the 0.1 Ce catalyst (the better synthesized material). The catalyst stability is essential for industrial applications since it has to maintain its activity during a long period of operation. One of the main drawbacks of metallic catalysts is the leaching of the active phase into the liquid phase (Martins et al. 2010). The concentrations of Ti and Ce, after the utilization of the catalyst, were determined in the reaction mixture. For both metals, the values were under the detection limits, 5 ppb (for Ti) and 70 ppt (for Ce). Then, the photocatalytic activity could be considered as a heterogeneous process. For the three light sources, the degradation of phenol (with 0.1Ce) followed an exponential trend. The apparent rate constants were (7.0±0.3) 10−4 s−1 (r2=0.99), (9.6±0.1) 10−4 s−1

[phenol] /  M

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Environ Sci Pollut Res

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time / minutes 0 350nm

white

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Fig. 6 Percentage of phenol degradation with the prepared catalysts, after 3 h (with 350 nm lamp) and 5 h (with white and 575 nm lamps)

Fig. 7 Phenol degradation with undoped (a) and 0.1 Ce (b) catalysts, after 3 h with 350 nm lamps (white circle, left axis) and intermediates detected (right axis): hydroquinone (black circle), catechol (black square), and p-benzoquinone (black triangle). [phenol]0 = 53 μM, 1 g L−1 of catalyst, air-saturated mixtures


(r2=0.99), and (9.1±0.6) 10−3 s−1 (r2=0.98) for white, 575 and 350 nm lamps, respectively. This observation is in line with reported behaviors for pristine anatase TiO2 (Wang et al. 2014) and Degussa P25 TiO2 with 80 % A and 20 % R (Peiró et al. 2001). However, the degradation of phenol by undoped TiO2 (calcined at 600 °C) could be described as biexponential decay, indicating a more complex mechanism. The absence of the A phase in this material (it is only R) may be responsible for this difference. For undoped and 0.1 Ce catalysts with 350 nm irradiation, three main intermediates were detected: hydroquinone, catechol, and p-benzoquinone, all reported as oxidative products of phenol (see Fig. 7). For pure TiO2 (in the form of A or a mixture of A and R) excited by UV irradiation, OH radicals are a primary oxidation species responsible for phenol degradation or mineralization. The attack of the electrophilic radical OH occurs at the ring positions activated by the presence of the substituent. The phenolic –OH group is electron-donating for the electrophilic aromatic substitution, then the electron density at the ortho and para positions increases. Hydroquinone and catechol are the main intermediates from this attack (Di Paola et al. 2003). The p-benzoquinone can be formed in three different ways: (i) by OH attack on the hydroquinone molecule; (ii) in the reaction of that molecule with holes photogenerated in titanium dioxide; and (iii) by oxidation of hydroquinone by oxygen dissolved in water (Grabowska et al. 2012). Catechol, similarly to hydroquinone, forms o-benzoquinone. The compound is, however, very unstable and hence its absence in the detected products of photochemical oxidation of phenol or catechol (Grabowska et al. 2012). In our experiments with the undoped catalyst, the amount of hydroquinone was 7.6 %, that of catechol, 5.3 % and pbenzoquinone, 1.4 % (assuming 100 % for initial phenol concentration). The presence of both dihydroxylated intermediates indicates the OH radical attack, as expected. In the case of metal-doped TiO2 excited by visible light, both OH radicals and direct oxidation could be employed (Grabowska et al. 2012). Moreover, lanthanide interacts strongly with the aromatic ring, and this interaction encourages the direct oxidation through the positive hole present on the surface (Silva et al. 2009). However, no significant adsorption of phenol on the 0.1 Ce catalyst was observed, making the contribution of direct oxidation in our studies less important. In the experiments with 0.1 Ce, a pronounced increase in the degradation rate of phenol was observed. Since cerium extends the photoresponse into the visible region, this can lead to an increase in the charge separation efficiency of surface electron-hole pairs. The redox pair of cerium (Ce3+/Ce4+) is also important, since cerium could act as an effective electron scavenger to trap the bulk electrons in TiO2 (Silva et al. 2009).

Under 350 nm irradiation, the utilization of 0.1 Ce catalyst yielded hydroquinone as the main intermediate (10.5 %, assuming 100 % for initial phenol concentration). Then, we propose the OH radicals attack phenol and the primary intermediates. As a consequence of these attacks, only a residual concentration of hydroquinone was detected after 3 h of treatment.

Conclusion It can be concluded that the increasing level of cerium in the material (i) increases the transition temperature for anatase to rutile phase transformation, (ii) confines the crystallite size of anatase phase, (iii) increases the specific surface area, (iv) decreases the crystallinity, and (v) induces a red shift of the electronic absorption band. However, no direct correlation between these characteristics and the photocatalytic activity was found. The material with higher efficiency corresponds to 0.1 Ce nominal at.%. Under irradiation with 350 nm lamps, the degradation of phenol could be described as an exponential trend, with an apparent rate constant of (9.1 ± 0.6) 10 − 3 s − 1 (r 2 = 0.98). Hydroquinone was identified as the main intermediate. Acknowledgments This work was supported by Grant PICT 2011 0832 from Agencia Nacional de Promoción Científica y Tecnológica, (ANPCyT, Argentina). J.A.R. and P.I.V. are research members of CONICET (Consejo Nacional de Investigaciones Científicas y Tecnológicas de la República Argentina). M.V.M. thanks CONICET for a postdoctoral studentship. The authors wish to thank E. Soto for his experimental contribution for N2physisorption measurements.

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